A mutant of Kirsten murine sarcoma virus has been isolated which is temperature sensitive for the maintenance of transformation. The p21 protein coded for by Kirsten virus has been shown to be thermolabile in this mutant strain of Ki-MSV. The results strongly suggest that the p21 is the src gene product of Ki-MSV and is therefore the protein responsible for Ki-MSV induced transformation. The mechanism by which this protein acts is being investigated and the protein is being purified.